SDF official Redur Xelil said: "The group was preparing to carry out terrorist attacks against the SDF. Also, we believe that the group was behind some of the attacks carried out against the SDF in recent days."

Among those captured were a 31-year-old
German , reportedly from Dortmund, and a 16-year-old American. Other fighters captured were Russian, Ukrainian, Kazakh, Tajik and Uzbek nationals.

German prosecutors reportedly knew of the Geman, identified as Lukas G. in line with German privacy laws, as having been radicalized in Dortmund. Der Spiegel news magazine reported that security agencies lost track of him and that he traveled to Syria in 2014, according to information provided to the Federal Criminal Police (BKA). Prosecutors started an investigation into G. in October 2016, suspecting him of being a member of a foreign terrorist organization. Officials at the time had no indication G. had a high position within IS.

Attack, counterattack

The YPG, which is part of an American-backed coalition fighting IS, is currently seeking to defeat the last pockets of Islamist militants active near the Iraqi border in eastern Syria. Last week IS militants launched a series of counterattacks aimed at SDF. Some 23 SDF fighters were killed in those attacks and 30 more were injured.

In all, at least 1,087 jihadis, 602 SDF fighters and 367 civilians have been killed since the SDF began its most recent offensive last September.

On Sunday, the YPG said it had captured five fighters, including two Americans, one of whom was a former teacher from Texas.

Thousands of foreign nationals captured

The SDF, which has been backed by coalition airstrikes, says it has captured roughly 1,000 foreign jihadis along the eastern banks of the Euphrates, as well as 550 foreign women and 1,200 children who were living with them.

The fate of the Kurdish fighters was recently cast into doubt when US President Donald Trump announced the immediate withdrawal of US troops from Syria. The announcement came at the same time Turkish President Recep Tayyip Erdogan said Turkish forces would launch a military offensive against the Kurds, which Ankara considers terrorists due to their ties to the Kurdistan Workers' Party (PKK).